1. Field of the Invention
The present invention relates to devices having glass engaging surfaces of a special composition for handling hot glass sheets. One embodiment of this invention comprises a device for transferring hot glass sheets from a shaping station to a cooling station, and particularly relates to the type of transfer device that returns empty to the shaping station for transferrring a succeeding hot glass sheet to the cooling station to perform a mass production method for shaping and heat-strengthening or tempering glass sheets. Other embodiments of this invention relate to rolls having glass sheet engaging surfaces composed of a special composition used to convey glass sheets through a moderately hot atmosphere, rolls having glass sheet engaging surfaces of said special composition to shape glass sheets passing thereover while the rolls rotate to move contacted heat-softened glass sheets and transfer rolls or roll segments having glass sheet engaging surfaces of said special composition that directly contact hot glass sheets during their handling. Still another embodiment of this invention involves a glass sheet lifting member that has a glass sheet engaging surface composed of said special composition. The present invention comprises methods of handling glass sheets involving directly contacting each sheet to be handled with a glass sheet handling device having an exposed surface composed of a special material that is both durable and that does not mar hot glass surfaces on engagement at elevated temperatures experienced during such handling.
2. Technology Problems and Patents of Interest
The prior art has provided devices for transferring hot glass sheet which comprise an outline or ring-type mold of relatively rigid metal that comprises an outline upper surface shaped to conform to the shape desired for the glass sheet and to which the glass sheet sags to conform by heating the glass-laden mold until the glass sheet is sufficiently soft to sag and conform to the shape provided by the shaping surface. When the glass sheet reaches the shaping surface, it is quenched suddenly and rapidly by applying cold air blasts against the supported lower surface and against the upper exposed surface of the glass sheet.
When the sagged glass sheet contacts the hot, heavy metal rail during this rapid cooling, glass breakage can result from surface vents at the glass-metal contact areas. In the past, glass sheet tempering molds of the outline type have been reduced in thickness along their glass engaging upper edge to reduce the effect of heat transferred at the glass engaging surface of the mold. As an alternative, tempering molds of the outline type have been provided with ribbons of insulating material such as asbestos or of heat-resisting material such as a strip of fiber glass. These ribbons space the supported glass sheet from direct contact with the heavy metal of the outline mold.
In recent years, glass sheets of thinner and thinner dimensions have been shaped and strengthened by rapid cooling to impart at least a partial temper. Such glass sheets have been shaped by lifting a glass sheet from a conveyor, which may be either of the roller type or of the gas hearth type, into engagement with a flat or contoured or deformable vacuum holder in vertically spaced relation to the conveyor. A ring-like member having an outline shape conforming to that of the glass sheet receives the shaped glass sheet from the vacuum holder and transfers the sheet to a cooling station. Preferably, the prior art ring-like member comprises one or more bars of nonmetallic material having a low heat transfer coefficient that engage the glass sheet. A metal ring hugs the bar or bars to reinforce the latter in spaced relation to the glass sheet engaging surface of the bar or bars so as to minimize the effect to the reinforcing metal ring on the rate the glass portion in contact with the nonmetallic bar or bars is cooled during rapid chilling.
Prior art ring-like members of the aforesaid construction have been used in combination with several types of shaping stations. In one type, a hot, deformable glass sheet is lifted to conform to the shape of a contoured vacuum holder prior to its transfer to the ring-like member. In a second type, a hot deformable glass sheet engages a deformable vacuum holder while flat and changes shape while the deformable vacuum holder is distorted. In a third type known as drop forming, a hot glass sheet is dropped from a flat or contoured vacuum holder onto a ring-like member of contoured outline so that it assumes the shape of the ring-like member on impact therewith as the still hot glass sheet drops from the vacuum holder onto the ring-like member. The latter then also serves as a transfer device to support the glass sheet during its conveyance from any of the aforesaid types of shaping stations to a cooling station.
In the past, the best material used for the transfer device has been able to maintain its initial shape when exposed to a limited elevated temperature range only. This has necessitated limiting the proportion of the cycle time for a transfer operation when the transfer device engages a hot glass sheet for transfer from the shaping station to the cooling station to limit the maximum temperature that the transfer device attains during a repetitive transfer cycle. A considerable proportion of the cycle time involved in transferring sucessive glass sheets on a transfer device from a shaping station to a cooling station had to be wasted while the transfer device was empty for a sufficient proportion of the cycle time to enable its temperature to recover to a temperature range sufficiently cool to avoid thermal damage of the nonmetallic bar or bars that comprises the glass engaging member of the transfer device. Furthermore, it was found that materials that did not mark the surface of the glass sheets at slow production rates marked the glass surface when production rates increased. Other materials used in the past wore unevenly and relatively rapidly at the higher temperatures developed during intermittent exposure to hot glass at more rapid production rates. The latter materials, on developing higher temperatures, needed reshaping or frequent replacement in order to maintain the shape of the glass sheets within desired specifications.
Donut type rolls have been used to transfer glass sheets from a ring-like member to an unloading conveyor, and shaped conveyor rolls composed of interfitting segments loosely fitted together to assume the shape of a curved shaft have been used in glass sheet handling. Glass sheets have been shaped while hot by conveying the glass sheets from a hot atmosphere into a shaping station where one or more glass sheets are lifted on shaped molds of spaced slats having shaped upper edges. Prior to the present invention, materials used in glass engaging locations either had limited utility at elevated temperatures or were carcinogenic in nature or were thought to require a coating of a special material that serves both as a solid film lubricant or glass release agent as well as a filler throughout the body of the glass engaging member.
It would be desirable for the glass sheet treating art to discover a material that can be used at higher temperatures than the materials previously used without sustaining thermal damage. It would also be desirable if the material is capable of use in a high speed operation at elevated temperatures without marking the shaped glass sheet surface when the sheets are transferred from a tunnel-type furnace or a shaping station to a cooling station while supported on a transfer device that contacts the glass sheet directly during its transfer. It would also be desirable to employ a high temperature material that does not mar a glass sheet it engages and is also safe for personnel handling.
U.S. Pat. No. 3,973,943 to Samuel L. Seymour discloses an outline ring-like transfer device for supporting shaped glass sheets after they are shaped and during the time they are rapidly quenched, comprising a rigid outline metal rail having an outline slightly inboard that of the shaped glass sheet and a bar or a plurality of closely spaced bar members of nonmetallic material having a lower heat transfer coefficient than that of the rigid metal rail and mounted in hugging relation against the rigid metal rail in position to conform to the outline and with an upper edge portion of the bar or bar members disposed above the upper edge of the rigid metal rail in position to provide a glass sheet supporting surface in spaced relation above the upper edge of the rigid metal rail. The bar or bar members of the Seymour patent are composed of a laminated structure, such as fiber glass cloth or asbestos woven paper layers or the like bonded together with a silicone resin binder or a phenolic resin binder. A device to unload the ring-like member of the Seymour patent using rotatable transfer rolls is shown in U.S. Pat. No. 4,092,141 to Robert G. Frank and Dewitt W. Lampman.
Prior to the present invention, the material most used for the glass engaging elements of the ring-like members of the Seymour patent and of the glass engaging surfaces of the ring-like rolls of the Frank and Lampman patent were composed of a material sold under the trademark SYNTHANE G-7 by Synthane-Taylor, an Alco Standard Company. This material was rated for maximum constant operating temperatures at 460.degree. F. Unfortunately, in order for such material to avoid marking the glass surface during its transfer, it was necessary to operate the transfer device transferring a shaped glass sheet to a cooling station in such a manner that the transfer device remained empty at least 50 percent of the cycle time for transferring a glass sheet from the shaping station to the cooling station and returning the transfer device from the cooling station back to the shaping station so that the transfer device could cool to a temperature it could withstand before it engaged the next glass sheet. This limited the maximum production rate possible.
When attempts were made to accelerate the rate of production using the transfer device of the aforesaid Seymour patent or the Frank and Lampman patent, the material engaging the glass caused the surface of the glass to become discolored. It became necessary to develop a transfer device having a glass engaging member composed of a material that did not mar the surface of the glass at the higher temperature range the glass engaging member of the transfer device developed because of the greater proportion of the cycle time in which the transfer device was in contact with the hot glass. The material also had to be sufficiently resistant to thermal damage at the higher temperature range of exposure of the transfer device developed at the higher production rate desired.
Other glass contacting elements used to handle hot glass sheets susceptible to surface damage on engagement therewith at various temperature ranges included certain conveyor rolls, transfer rolls and forming rolls which altered the shape of moving glass sheets passing thereover and a glass sheet shaping station located in the vicinity of a furnace exit.
The search for high temperature resistant compositions for engaging glass at high temperatures led to a discovery of a composite material reported in U.S. Pat. No. 4,246,313 to Edward J. Stengle, Jr. The composite material of this patent comprises a substrate of a polymeric thermoset material such as polyaramid fibers or similar high temperature resistant material having a continuous coating or layer of solid film lubricant or glass release agent which essentially comprises an organic-inorganic silicone resin having a finely divided particulate filler therein. The polymeric fibrous substrate of this patent cannot be employed alone without a suitable heat-resisting coating for long-term repeated handling of hot glass articles. However, the coated surface of the materials covered in this patent serves as a film lubricant that allows glass slippage. Consequently, the materials covered in this patent are not suitable for use in handling glass sheets for certain treatments such as shaping or applying a design to the sheets, that require the sheets to be aligned and/or oriented precisely for the particular treatment.